The applicants plan to evaluate the turnover of naive (CD45 RA) and memory (CD45RO) CD4 and CD8 lymphocytes in HIV-infected children and adolescents using several methods, specifically: DNA labeling with stable-isotope labeled glucose; telomere length; and, cell cycle/apoptosis by flow cytometry. Data will be obtained before and after 20 and 48 weeks of highly active antiretroviral therapy. Age- matched uninfected children will be tested for comparison. To define better the role of the thymus in turnover of naive and memory CD4 and CD8 cells, children who as infants underwent thymectomy during cardiac surgery and age-matched healthy children will be studied using the same methods as those used for HIV-infected children. The applicant states that these studies will provide information on 1) the role of the thymus in T cell lymphocyte homeostasis at different ages, 2) the effect of HIV infection on cell turnover and, 3) the effect of HAART on T cell turnover in HIV-infected children and adolescents.